Mobile communications systems are known in the art for providing a communications link between a mobile vehicle (e.g., automobile, truck, train, airplane or the like) and stationary base or another mobile vehicle. Communications link, as used in the present application is defined, but not limited to voice, data, facsimile or video transmission or the like. Some such known systems utilize local radio transmitters and receivers, for example, various radio dispatched vehicles (taxis, police, deliveries, repair services, or the like) ham or amateur radio, Citizens Band Radio (CB), commercial transmitters, cellular systems or the like.
The disadvantage of these local radio frequency devices is that they provide only a limited scope of coverage. Practical limitations in transmitter and receiver design as well as bandwidth considerations limit the range of such systems. For some applications, for example, commercial transportation (e.g., shipping; common carriers and the like) it is desirable to provide communications coverage for a larger area, such as the continental United States (CONUS). Such coverage is possible with a series of local transmitter stations strategically located throughout the CONUS area, however, the practical limitations of maintaining and operating such a large number of transmitting stations renders such a system too costly and impractical. Further, even if such a system were implemented, coverage over the entire CONUS could not be assured, as "blackout" areas could arise due to local terrain and weather conditions.
As such, it has been proposed to provide a Mobile Satellite Communications system (MSAT) for use in providing a communications link between one or more stationary bases and mobile vehicles, or between stationary bases or between mobile vehicles. Satellite communications systems are known in the art and have been extensively used in the telecommunications and television arts. For example, a satellite can be placed in a geosynchronous and/or geostable orbit with a broadcast "footprint" which covers the entire CONUS. Of course, other "footprint" sizes could also be used to cover other geographic areas. Further, multiple satellites could also be used to provide a plurality of "footprints" (overlapping or not) to cover a particular area or areas.
The use of a satellite system overcomes many of the disadvantages of local radio frequency networks. For example, it is possible, with a satellite system, to use one satellite transponder to provide a common data link with a plurality of vehicles or sites throughout the CONUS. The use of new, so-called "high power" satellite transponders in higher frequency bands (e.g., Ku-band, L-band and the like) makes possible a more robust, stronger signal which can be more readily received throughout the entire CONUS.
Such a strong signal is desirable in mobile applications in particular as constraints are placed on antenna design. For example, in early telecommunications and television applications, so called "low" power satellite transponders (on-the order of tens of watts) provided a fairly weak signal which generally required a fairly large antenna to receive. Typical terrestrial antennas were parabolic designs (or variants thereof) on the order of at least a meter or more in diameter, utilizing low noise amplifiers to amplify the relatively weak received signal.
For mobile applications, a more compact, relatively omni-directional antenna is desirable. Aerodynamic and aesthetic requirements necessitate that the antenna design be small and relatively short. Further, the antenna must also be robust in order to survive in a mobile (e.g., automotive) environment. In addition, if such a system is to be widely adopted, the antenna design must be relatively inexpensive in order to keep the overall cost of the mobile transceiver down. Since the communications link between the satellite and the antenna is more or less a line of sight transmission link and since a mobile vehicle is rarely positioned in one location for any given period of time, an efficient, relatively omni-directional antenna is needed.
Thus, prior art parabolic antenna designs are impractical for mobile use. Such antennas are relatively large and expensive and largely unidirectional. For mobile applications, an antenna positioning device would be needed to constantly reposition the antenna for optimum reception. Furthermore, such an antenna design would be much too bulky for mobile application, presenting too large a surface for aerodynamic considerations, and presenting a generally displeasing aesthetic appearance. Moreover, in mobile applications, such an antenna design would be too delicate to survive long. Low hanging branches, parking garages and other aerial hazards would quickly destroy such a large antenna.
An example of one such mobile parabolic dish design is shown in Suzuki et al. U.S. Pat. No. 4,725,843, issued Feb. 16, 1988 shown in FIG. 1. FIG. 1 shows a vehicle 3 with parabolic dish antenna 1 and feed horn 2. As can be readily ascertained from FIG. 1, the relatively large dish antenna 1 precludes the use of any rooftop accessories (e.g., roof rack or the like) and presents quite a profile to the wind. In addition, such a design is somewhat aesthetically displeasing, thus precluding mass consumer acceptance. Such mobile satellite communications systems have consumer applications and as such, a pleasing aesthetic design is a necessary criteria. The parabolic dish 1 of FIG. 1 also requires a positioning mechanism to constantly reposition dish 1 as vehicle 3 travels. Such a positioning system is complex and fragile, adding to the cost and maintenance of the unit and detracting from the reliability and robustness of the design. Finally it is noticeable that the design of FIG. 1 is particularly susceptible to damage due to low clearances such as garages and the like.
A practical MSAT antenna must also be able to compensate for changes in latitude. In particular, as a vehicle travels from areas of high latitude (e.g., Northern CONUS) to areas of lower latitude (e.g., Southern CONUS), the angle of elevation between the vehicle and the satellite changes (e.g., from 20.degree. to 60.degree.). Thus it remains a requirement to provide an antenna which, although maintaining relatively omni-directional coverage in the azimuth, is capable of scanning its main radiation beam in elevation to compensation for changes in latitude.
For applications in which it is desirable to provide both transmit and receive capabilities in the mobile unit, the antenna must also be able to efficiently transmit radio signals to the satellite and receive return signals as well. In typical radio communications systems, different frequencies are chosen for the transmit and receive signals in order to prevent interference between these two signals. Unfortunately, most antenna designs are optimized for one frequency or a range or band of frequencies. As with all travelling wave antennas, the location of the peak radiation beam varies with frequency, giving rise to a phenomenon called "frequency scanning". This phenomena results in an unfortunate reduction in antenna gain between the transmit and receiving modes of operation. This reduction in gain is sometimes called "cross-over loss".
Thus, it remains a requirement in the art to provide a small, inexpensive, efficient vehicular MSAT antenna which has relatively omni-directional coverage in azimuth. It remains a further requirement in the art to provide an MSAT antenna which has an aesthetically pleasing and robust design. It remains a further requirement in the art to provide an MSAT antenna which is capable of scanning its main radiation beam in elevation while remaining relatively omni-directional in azimuth. It remains an even further requirement in the art to provide a vehicular MSAT antenna with reduced frequency scanning.
The present invention solves these and other problems by providing a multi-turn quadrifilar helix antenna fed in phase rotation at its base. The antenna of the present invention provides for an adjustment of the helix elements, causing beam scanning in the elevation plane. The quadrifilar helical antenna is omni-directional in azimuth, making the antenna particularly suitable for a mobile vehicular antenna accessing stationary satellites.